Tonight, a baffling portrait is emerging of that mass shooter oregon who opened fire on a crowd shopping for some holiday cheer. This year, there's been a disturbing number of mass shootings, and investigators say while copycats may be inspired, each shooter has a distinct and chilling back story. We interviewed the 22-year-old's ex-girlfriend, who calls it mind boggling. Reporter: Chaos amidst the christmas mall rush. Lines of shoppers, even children could be seen streaming out with their hands up while a gunman was still inside, announcing from behind a mask, "i am the shooter." A day later, a view of the suspect is now coming into focus. Police say 22-year-old jacob tyler roberts was the masked gunman, a young man with no history of serious trouble, who allegedly stole an assault rifle from an acquaintance and set off alone on a deadly rampage. We do not understand the motive of this attack, except to say that there's no apparent relationship between the suspect and his victims. Reporter: Hannah sansburn doesn't understand either. This is the last thing i would have ever expected. Especially from him. He was just too sweet. Reporter: In an interview with abc news, the shooter's ex-girlfriend told us she has no idea what triggered the sudden andrutal attack. That's what is so mind boggling, because it is -- that was not the person I knew. And the person I knew would never, ever have done anything like this. Reporter: But there was something strange in recent weeks. He says he seemed numb when she saw him last week, but she didn't think much of it. And she says he suddenly quit his job at a greek deli and sold all of his belongings, telling her he was leaving portland and moving to hawaii this past weekend. He had his plane ticket. He was ready to go. And then this happened, and it just makes me think why it was even in the back of his mind or if he just had another plan to leave. Reporter: Instead of hawaii, police say he went to the mall dressed to kill. Police say roberts made a beeline from his volkswagen jetta in the parking lot through the macy's store carryin a rifle, wearing a ballistics vest, and a hockey style facemask. He entered on the upper level, when he got to the food court, he was already reportedly firing, killing 54 hospice nurse cindy ann yuille, who was christmas shopping, and 45-year-old steven forsyth, a father of two, who owned a small business in the mall. He fired into the lower level, hitting this 15-year-old, now in the hospital. There his gun jammed. He bolted down a flight of stairs where he got his rifle working again, according to police, and killed himself near the jcpenney store. It was just the latest rampage in a violent year. July's massacre in aurora, colorado, killed 12, but it wasn't the only mass tragedy. APRIL 2nd, SEVEN DEAD ON A University campus in oakland, california. In august and september, seven dead at a sikh temple in milwaukee, then six dead at a workplace in minneapolis. The ability to shoot and kill other human beings, to actually pull the trigger, requires the shooter look at these humans more as objects thanther humans. When you're able to do that, it makes pulling that trigger so much easier. Reporter: Veteran fbi profiler mary ellen o'toole says the triggers for such random violence are not black and white. These shooters -- yes, can they have mental health issues? Absolutely. But is the reason they're carrying out these crimes because they are mentally ill? I would say no. It's important to make that distinction because most people who are seriously mentally ill are far less violent than the general population. Reporter: And you had never known him to want to hurt anyone? Never. Never. He never would hurt anybody. I mean, the only way that he would ever get him to be physically physical with you is if you're tickling him. Reporter: Today, the woman who raised roberts released this picture of the once happy little boy, who police say somewhere along the way turned from happy go lucky to dangerous and unpredictable. A family friend read a statement. She has no understanding or explanation for her son's behavior. She's very sad and wants everyone to know she is so sorry what jake did and it's so out of his character. Reporter: Tonight police are running search warrants and chasing down leads. Trying to find an explanation for an act of violence that makes no sense. Coming up for us, naomi

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